The invention relates to vibration control systems for attenuating the vibration of a machine having moving parts.
In order to impart motion to one or more parts within a machine, it is necessary to apply a force to these parts. Basic physics explains that a force cannot be simply applied to a single body, but must be applied between two bodies. Hence, the application of a force to impart motion to a body within a machine simultaneously exerts an opposite force on the remainder of the machine. Consequently, the reactive forces on the machine cause the machine to vibrate.
In the past, many methods have been proposed for controlling and reducing the level of undesirable mechanical vibrations in machines. Some of these methods are described in an article entitled "Fundamental Concepts of Vibration Control" by Jerome E. Ruzicka (Sound and Vibration, July 1971, pages 16-22).
For example, passive vibration isolation and absorption uses a combination of springs and dampers to isolate the vibrating body from its environment and to absorb and dissipate vibrational energy. Problems with this approach, however, include the fact that this technique cannot be used to simultaneously reduce the amplitude of the displacement of the machine due to the vibrations and the vibrational forces transmitted through the machine mount. Moreover, a machine with passive vibration isolation and absorption does not behave as a constant mechanical impedance (i.e. it does not behave as a solid body with constant mass). This latter factor can be a concern in aircraft and spacecraft applications.
Another method of vibration control is mechanical balancing. A rhombic drive is an example of this approach, where each moving mass is counteracted by a balancing countermass which is mechanically driven in opposition to the moving mass. Problems with this approach include the fact that balancing is limited by the accuracy, symmetry, and linearity of each compensating component. Further, mechanical balancing imposes undesireable design constraints, such as requiring significantly larger machine volumes and more moving parts. Moreover, the performance of this approach may seriously degrade with time or external influences.
Another related method of vibration control is passive vibration compensation, which uses inertial compensation through a resonant spring-countermass combination. This method is reasonably effective if the inertial force imbalance to be compensated is primarily sinusoidal at a single constant frequency. The spring-mass combination can be tuned to this frequency so that it responds to vibrations by oscillating to help cancel the vibrations. However, the effectiveness of this approach is limited because compensation only occurs at the single selected frequency, the amount of compensation depends upon the characteristics of the mechanical connection between the machine and its environment, and performance may seriously degrade with time or external influences. Moreover, as in the case of mechanical balancing, each moving body must usually be provided with a spring-countermass combination. This again results in larger machine volumes and more moving parts.
The method of vibration control most recently proposed to date utilizes active vibration isolation and absorption. In this system, a sensor is attached to the machine subject to vibration. The sensor produces an output signal which is proportional to the acceleration of the machine. The sensor output signal is processed by a signal processor to produce a control signal for driving a reaction mass in such a manner as to reduce the total acceleration of the machine. (See, for example, U.S. Pat. No. 4,083,433, and "Comparison of Optimized Active and Passive Vibration Absorbers," by J. Morison et al., Joint Automatic Control Conference of the American Automatic Control Council, 1973, pages 932-938.)
These new active vibration control systems are not entirely problem-free. Since the feedback signal comes from the gross motion of the machine, the net compensation is affected by the characteristics of the machine mount. The mechanical mount therefore limits performance and can also affect the stability of the feedback loop. Moreover, this scheme attempts to keep the machine in a single inertial fram of reference. This causes the mechanical impedance of the machine to vary both with frequency and with time. The vibration control system thus acts to oppose and counteract any movement of the machine, whether vibrational or translational. These problems can be especially important where the machine/vibration control system is to be used in aircraft or spacecraft.